Ideas
You thankfully have proven I am not an idiot. After many hours of changing key bindings and testing I finally figured out why flying helicopters in an arcade style game sucks so bad. I will also note that the attack helicopter does not have roll with assist on. When tipping nose up or down while yawing, it looks like there is roll. I saw some expert gamers playing and they definitely had rolling on the attack chopper but I cannot duplicate with assist on.
In my opinion, the assist should just keep the copter hovering and that is it. But to be honest, it is an arcade style shooter with realistic elements. Choppers should auto hover regardless.
I have not pinpointed anything yet but wonder what is screwed up on jets. I managed to figure out how to fly them but it still is wonky. I guess jet yaw is slight tuning of left/right to perfect your direction? It is so subtle in game I find it pointless.
P.S. I do not get why I don't see more comments or questions around the internet on this. The helicopter was designed for fast turns and hovering so why would a type in BF6 not have yaw?